‘ And Joshua said to Achan, “My son, give, I pray you, glory to YHWH, the God of Israel, and make confession to him, and tell me now what you have done. Do not hide it from me.” '
This was a stern legal adjuration. To ‘give glory to YHWH' in such circumstances was to be open with the truth (compare Jeremiah 13:16; John 9:24). He was to confess to YHWH by telling the judge. By doing so he would bring glory to YHWH whose representative the judge was. The whole truth was to be told. Nothing must be hidden.
Normally a man could not be adjured to condemn himself. But here Achan was already condemned because of his selection by YHWH. Whether he confessed or denied he would be executed. By admitting his fault he would be bringing glory to the One Who knew about his sin even before he admitted it.